Cooling unit with multi-parameter defrost control

ABSTRACT

A cooling unit apparatus and method for controlling a defrost cycle using a multi-parameter defrost control system. The unit includes a refrigeration assembly with a compressor and an evaporator, a defrost temperature sensor that senses a temperature related to the defrost cycle, a controller configured to control a defrost cycle based on an elapsed defrost time and the defrost temperature. The controller is programmed with a defrost set point and a maximum defrost time. The controller ends the defrost cycle when the elapsed defrost time is greater than the maximum defrost time or when the defrost temperature is greater than the defrost set point, whichever occurs first.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional patent application Ser. No. 60/862,376 filed on Oct. 20, 2006, and entitled “Cooling Unit,” hereby incorporated by reference as if fully set forth herein.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to refrigerated food and drink storage units, and in particular, to the controller therefore, and even more particularly to the control of a defrost cycle of a refrigeration system.

2. Description of the Related Art

Refrigerators and coolers for the cold storage of food and beverages are well known and can come in full-size standup units or compact, under-cabinet units.

Cooling units typically consist of an electrically driven compressor, a condenser, an evaporator and a flow-restricting device. The compressor inlet is connected to draw refrigerant from the evaporator, and the compressor outlet is connected to discharge the refrigerant under increased pressure and temperature to the condenser. Under such conditions, the hot refrigerant entering the condenser is cooled by external means, usually air or water, thereby extracting heat from the refrigerant. As the temperature of the refrigerant drops under substantially constant pressure, the refrigerant in the condenser liquefies, or condenses, thereby losing additional heat due to latent heat of vaporization for the refrigerant.

The flow restricting device, usually either a capillary tube or expansion valve, is connected between the condenser and evaporator so as to maintain the high pressure in the condenser and at the compressor outlet while simultaneously providing substantially reduced pressure in the evaporator. The substantially reduced pressure in the evaporator results in a large temperature drop and subsequent absorption of heat by the evaporator.

The very cold temperature of the evaporator can result in formation of frost thereon, which eventually results in the buildup of ice on the surface of the evaporator. As the ice continues to collect on the evaporator, the transfer of heat is reduced due to the insulative effect of the ice. Further, the ice can restrict the flow of air between evaporator fins. Consequently, in order to remove the accumulated ice, it is necessary to periodically defrost the evaporator. A common defrosting method is the use of an electrical resistance heater that heats the evaporator.

U.S. Pat. No. 5,065,584, hereby incorporated by reference as if fully set forth herein, teaches a hot gas defrost system including a hot gas bypass valve. Defrosting is achieved by the use of hot, uncondensed refrigerant, without requiring a reversal of the flow of refrigerant through the system. An exemplary bypass means includes a hot gas bypass solenoid valve and associated tubing to form an alternate, low restriction path for the refrigerant to bypass the capillary tube. When the solenoid valve is closed, refrigerant is forced through the capillary tube for normal refrigeration. However, when the solenoid valve is open, condensation of refrigerant in the condenser is inhibited, and hot refrigerant gas is delivered directly to the evaporator for defrosting. The solenoid valve is controlled in response to the total accumulated running time of the compressor for improved control of the defrosting cycles.

For cooling units without freezers that use plate evaporators, another defrost method is to simply shut down the refrigeration system for a period of time after a certain amount of compressor runtime. Any frost on the plate evaporator melts as the plate evaporator warms above the freezer temperature. Once defrost has been completed, the compressor is allowed to run to cool the unit.

Typically, defrost operation is executed irrespective of the quantity of the frost produced at an evaporator. However, the quantity of the frost produced at the evaporator depends on temperature, humidity about an evaporator and the quantity of the foods stored in the refrigerator. To ensure that the evaporator is properly defrosted, the length of the defrost cycle is typically set to cover the worst case. Regardless of the defrost method, the cooling unit can not be cooled during defrost, which means that the cooling unit can warm above operational temperatures during defrost thereby negatively impacting temperature control. Thus, the defrost cycle should provide for adequate defrost while minimizing the defrost time.

SUMMARY OF THE INVENTION

The invention provides a cooling unit with a multi-parameter defrost control system. The cooling unit includes a refrigeration assembly including a compressor and an evaporator and a defrost temperature sensor that senses a defrost temperature and a controller. The controller is configured to control a defrost cycle of the refrigeration system, track an elapsed defrost time of the defrost cycle and monitor the defrost temperature, the controller programmed with a defrost set point and a maximum defrost time. The controller is configured to end the defrost cycle when the elapsed defrost time reaches the maximum defrost time or when the defrost temperature reaches the defrost set point, whichever occurs first.

The cooling unit can include a user input connected to the controller. The defrost set point, the maximum defrost time and the defrost interval period can be set by the user input.

The refrigeration assembly can include a hot gas bypass valve configured to allow heated refrigerant to heat the evaporator when the controller opens the hot gas bypass valve during the defrost cycle. The refrigeration assembly can also include an evaporator pan and an evaporator pan heater that is energized by the controller during the defrost cycle to heat the evaporator pan. A defrost temperature sensor can be in thermal communication with the evaporator so that the defrost temperature indicates a temperature of the evaporator. A defrost temperature sensor can be in thermal communication with the evaporator pan so that the defrost temperature indicates a temperature of the evaporator pan. The defrost set point can be 42 degrees Fahrenheit. The controller can be configured to so that the controller does not end the defrost cycle when the elapsed defrost time is less than a minimum defrost time.

The evaporator can be a plate evaporator and the defrost temperature sensor can be in thermal communication with the plate evaporator so that the defrost temperature indicates a temperature of the plate evaporator. The defrost set point can be 42 degrees Fahrenheit.

In another aspect, the present invention provides a method for controlling a defrost cycle of a cooling unit with a refrigeration assembly including a compressor and an evaporator, a defrost temperature sensor that senses a defrost temperature, and a controller programmed with a defrost set point and a maximum defrost time. The method includes monitoring the defrost temperature, tracking an elapsed defrost time of the defrost cycle beginning at a start time of the defrost cycle, ending the defrost cycle when the elapsed defrost time reaches the maximum defrost time and ending the defrost cycle when the defrost temperature reaches the defrost set point.

The refrigeration assembly can include a hot gas bypass valve configured to allow heated refrigerant to heat the evaporator when the controller opens the hot gas bypass valve during the defrost cycle. The refrigeration assembly can further include an evaporator pan and an evaporator pan heater that is energized during the defrost cycle to heat the evaporator pan. The defrost temperature sensor can be in thermal communication with the evaporator pan so that the defrost temperature indicates a temperature of the evaporator pan. The defrost set point can be 42 degrees Fahrenheit.

The evaporator can be a plate evaporator that is defrosted when the refrigeration system is not energized.

Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a combination refrigerator/freezer unit having the features of the present invention;

FIG. 2 is a perspective view thereof similar to FIG. 1 albeit with its cabinet door open so that the interior of the cabinet is visible;

FIG. 3 is a front elevation view thereof with the cabinet door removed;

FIG. 4 is an exploded assembly view thereof;

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3;

FIG. 6 is a diagram of the refrigeration system of the combination unit of FIG. 1;

FIG. 7 is a schematic of the control system of the combination unit of FIG. 1;

FIG. 8 is an exploded view of a refrigerator unit having the features of the present invention;

FIG. 9 is a diagram of the refrigeration system of the combination unit of FIG. 8;

FIG. 10 is a schematic of the control system of the combination unit of FIG. 8;

FIG. 11 is a defrost time decision flow chart; and

FIG. 12 is the defrost time decision flow chart with an additional decision block.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1-4, a combination refrigerator/freezer unit 30 includes a cabinet 32 defining a cavity with a forward opening 34 that is divided by horizontal and vertical partition walls 36 and 38, respectively, into a refrigerator section 40 and a freezer section 42. The refrigerator section 40 is an L-shaped chamber having a molded insert liner 44 with grooves that support shelves 46 (two are shown in the drawings). The shelves 46 are supported by corresponding grooves formed in the vertical partition wall 38. Molded insert liner 44 includes a pair of grooves that support a lower support shelf 48 and defines a recess for a crisper drawer 50. The freezer section 42 is a rectangular chamber having a foam insulated, molded insert 52 containing a cube ice maker assembly 56 and an ice storage bin 58. The freezer section 42 is closed by a door 60 that is hinged to insert 52 along one vertical side thereof. The cabinet opening 34 is closed by a door 64 that is hinged to the cabinet 32 (with self-closing cams) along one vertical side thereof. Both the cabinet 32 and the door 64 are formed of inner molded plastic members and outer formed metal members with the space filled in with an insulating layer of foam material, all of which is well known in the art. The door 64 has a handle 61 and can include one or more door shelves.

Along the back wall of the freezer section 42 is an evaporator 62 with serpentine refrigerant tubes running through thin metal fins. The evaporator 62 is part of the refrigeration system of the unit 30. With reference to FIGS. 4 and 6, the evaporator 62 has an outlet line 66 which is connected to the inlet of a compressor 70. A discharge line 72 connected to the outlet of the compressor 70 is connected to the inlet of a condenser 74 having an outlet line 76 connected to a dryer 78. A capillary tube 80 leads from the dryer 78 an inlet line 82 of the evaporator 62. A bypass line 84 leads from the dryer 78 to the inlet line 82 of the evaporator. A hot gas bypass valve 86 controls communication between the dryer 78 and the evaporator 62. Bypass valve 86 can be an electronically controlled solenoid type valve. An evaporator fan 89 (see FIG. 3) is positioned near the evaporator 62 and a condenser fan 90 (see FIG. 7) is positioned near the condenser 74. An evaporator cover 91 covers the evaporator 62. An evaporator pan 92 is positioned beneath the evaporator 62 and is configured to collect and drain water. An evaporator pan heater 94 is beneath the evaporator pan 92 to heat the evaporator pan 92. The compressor 70, condenser 74 and condenser fan (see FIG. 7) are located at the bottom of the cabinet 32 below the insulated portion.

Referring now to FIGS. 4 and 7, a controller 128 is attached below the cabinet and adjacent a kickplate 130 positioned below the cabinet door 64. The controller 128 comprises a microprocessor (not shown) that is connected to a memory (not shown). Alternatively, the microprocessor can include a memory. A plurality of connectors and lines (not shown) connect the controller 128 to sensors (discussed below) and relays associated with the other electrical components (not shown) of the refrigeration unit 30. A refrigerator section temperature sensor 138 is attached to refrigerator section 40 (see FIGS. 3 and 4) and senses the temperature of refrigerator section and provides refrigerator section temperature information to the controller 128. A freezer section temperature sensor 140 is attached to the freezer section 42 (see FIGS. 3 and 4) and senses the temperature of the freezer section 42 and provides freezer section temperature information to the controller 128. An evaporator pan temperature sensor 142 is attached to the evaporator pan 92 (see FIG. 4) and senses the temperature of the evaporator pan 92 and provides evaporator pan temperature information to the controller 128. The temperature sensors 138, 140, and 142 can comprise thermistors or other appropriate temperature sensors. The controller 128 is configured to control refrigeration, ice making, defrost and other aspects of the refrigeration unit 30 as will be described below.

As is known, the compressor 70 draws refrigerant from the evaporator 62 and discharges the refrigerant under increased pressure and temperature to the condenser 74. The hot, pre-condensed refrigerant gas entering the condenser 74 is cooled by air circulated by the condenser fan 90. As the temperature of the refrigerant drops under substantially constant pressure, the refrigerant in the condenser 74 liquefies. The smaller diameter capillary tube 80 maintains the high pressure in the condenser 74 and at the compressor outlet while providing substantially reduced pressure in the evaporator 62. The substantially reduced pressure in the evaporator 62 results in a large temperature drop and subsequent absorption of heat by the evaporator 62. The evaporator fan 89 can draw air from inside the freezer section 42 across the evaporator 62, the cooled air returning to the freezer section 42 to cool the freezer section 42. At least one air passage (not shown) connects the freezer section 42 and the refrigerator section 40 so that the refrigerator section 40 is cooled by the freezer section 42, the temperature of the refrigerator section 40 related to the temperature of the freezer section 42. The compressor 70, condenser fan 90 and evaporator fan 89 are controlled by the controller 128 to maintain the freezer section 42 at a freezer section set point. The freezer section set point is based on a refrigerator section set point (e.g., freezer section set point is the refrigerator section set point minus 30 degrees Fahrenheit), the refrigerator section set point being inputted by a user as described below. The controller 128 logs the compressor runtime between defrost cycles and stores the compressor runtime in the controller memory 134.

As mentioned, the refrigeration system includes hot gas bypass valve 86 disposed in bypass line 84 between the dryer 78 and the evaporator inlet line 82. Hot gas bypass valve 86 is controlled by controller 128. When the hot gas bypass valve 86 is opened, hot pre-condensed refrigerant will enter the evaporator 62, thereby heating the evaporator 62 and defrosting any ice buildup on the evaporator 62. The evaporator pan heater 94 heats the evaporator pan 92 when the hot gas bypass valve 86 is opened so that frost/ice in the evaporator pan 92 is melted at the same time that the evaporator 62 is defrosted in order to ensure that the water properly drains from the evaporator pan 92. The evaporator fan 89 is de-energized during the hot gas defrost cycle to prevent or limit heat rejection at the condenser 74. The hot gas bypass valve 86 and evaporator pan heater 94 are controlled by the controller 128 (i.e., the defrost cycle is controlled by the controller 128). The controller 128 logs the defrost runtime and stores the defrost runtime in the controller memory 134. The interval between defrost cycles can be adjusted by the controller 128. The controller 128 calls for a defrost cycle after a certain amount of compressor runtime (e.g., between six and twelve hours).

A period of time (e.g., five minutes) before the controller 128 starts the hot gas defrost cycle due to a call for a hot gas defrost cycle, the controller 128 energizes the evaporator pan heater 94. When the controller 128 starts the hot gas defrost cycle, the controller 128 opens the hot gas bypass valve 86 and continues to provide energy to the compressor 70 which is already energized. Thus, the evaporator 62 and evaporator pan 92 are heated to melt any frost or ice on the evaporator 62 and the evaporator pan 92. During the defrost cycle, the controller 128 monitors the temperature T_(PAN) of the evaporator pan temperature sensor 142 and keeps track of the elapsed defrost time D_(T). The hot gas defrost cycle continues until the evaporator pan temperature T_(PAN) reaches an evaporator pan set point (e.g., 42° F.) or the elapsed defrost time D_(T) reaches a maximum defrost time D_(T-MAX) (e.g., 45 minutes), whichever occurs first. This provides adequate defrosting of the evaporator 62 while minimizing the amount of defrost time and, thus, the amount of the time that the unit 30 is not able to be cooled by the refrigeration system. The hot gas defrost cycle is ended by closing the hot gas bypass valve 86 and turning off the compressor 70, or by just closing the hot gas bypass valve 86 if the controller 128 is calling for cooling after the defrost. The controller 128 continues to energize the evaporator pan heater 94 for a period of time (e.g., five minutes) after the hot gas defrost cycle has ended. Alternatively, the evaporator pan heater 94 can be energized for the same amount of time that the hot gas bypass valve 86 is open. The time that the evaporator pan heater 94 is energized is not included in the calculation of the elapsed defrost time. Additionally, the controller 128 can run the hot gas defrost cycle for a minimum amount of time (e.g., 18 minutes) before checking temperature T_(PAN) of the evaporator pan temperature sensor 142, which helps to prevent frost build up when the evaporator pan temperature sensor 142 is not functioning properly (e.g., reading a higher than actual temperature so that the defrost cycle would end prematurely).

Referring now to FIG. 3, a user interface control unit 160 is mounted to the top of the refrigerator molded insert liner 44 within the cabinet 32 for receiving user commands and forwarding input signals to the main controller 128. The control unit 160 includes a display panel 162 and an input control board (not shown). The input control board is connected to the controller 128 and relays command information from a user inputted on a power input 166, a warmer input 168, a cooler input 170 and a light input 172. The controller 128 and control unit 160 are configured so that a user can set the minimum defrost time, the evaporator pan set point, the maximum defrost time.

Referring now to FIG. 8, a refrigerator unit 300 includes a cabinet 302 defining a cavity with a forward opening 304. The cabinet 302 has a molded insert liner 306 with grooves that support shelves 308 (two are shown in the drawing). The shelves 308 are supported by corresponding grooves formed in the molded insert liner 306. The molded insert liner 306 also includes a pair of grooves that support a lower support shelf 310 and a crisper drawer 312. The cabinet opening 304 is closed by a door 314 that is hinged to insert 306 along one vertical side thereof. Both the cabinet 302 and the door 314 are formed of inner molded plastic members and outer formed metal members with the space filled in with an insulating layer of foam material, all of which is well known in the art. The door 314 has a handle (not shown) and includes shelves 316 (two are shown in the drawing).

Referring now to FIGS. 8-10, the unit 300 includes a refrigeration system 320. The refrigeration system 320 includes a plate evaporator 322 that is positioned along the back wall of the insert liner 306. The plate evaporator 322 fits along substantially the entire back wall of the insert liner 306 and has a large thermally conductive surface area. The plate evaporator 322 includes evaporator tubes 323 through which the refrigerant flows. The plate evaporator 322 has an outlet line 324 that is connected to the inlet of a compressor 326. A discharge line 328 connected to the outlet of the compressor 326 is connected to the inlet of a condenser 330 having an outlet line 332 connected to a dryer 334. A capillary tube 336 leads from the dryer to an inlet line 338 of the plate evaporator 322. A condenser fan 340 is positioned near the condenser 330. The compressor 326, condenser 330, and condenser fan 340 (see FIG. 8) are located at the bottom of the cabinet 302 below the insulated portion.

Referring now to FIG. 8, a controller 350 is attached below the cabinet 302 and adjacent a kickplate 352 positioned below the cabinet door 314. The controller 350 comprises a microprocessor (not shown) including memory or attached to a memory. A plurality of connectors and lines (not shown) connect the controller 350 to a sensor (discussed below) and relays associated with the other electrical components (not shown) of the unit 300. A plate evaporator temperature sensor 354 is attached to the plate evaporator 322 (see FIG. 11) and provides plate evaporator temperature information to the controller 350. A cabinet cavity temperature sensor (not shown) is attached to the liner 306 and provides temperature information about the cabinet cavity to the controller 350. The temperature sensor 354 and cabinet cavity temperature sensor can comprise thermistors or other appropriate temperature sensors. The controller 350 is configured to control refrigeration and defrost as will be discussed hereinafter.

As is known, the compressor 326 draws refrigerant from the plate evaporator 322 and discharges the refrigerant under increased pressure and temperature to the condenser 330. The hot, pre-condensed refrigerant gas entering the condenser 330 is cooled by air circulated by the condenser fan 340. As the temperature of the refrigerant drops under substantially constant pressure, the refrigerant in the condenser 330 liquefies. The smaller diameter capillary tube 336 maintains the high pressure in the condenser 330 and at the compressor outlet while providing substantially reduced pressure in the plate evaporator 322. The substantially reduced pressure in the plate evaporator 322 results in a large temperature drop and subsequent absorption of heat by the plate evaporator 322. The plate evaporator 322 absorbs heat from the interior of the cabinet 302 thereby cooling the unit 300. The compressor 326 and condenser fan 340 are controlled by the controller 350 to maintain the unit 300 at a set point. The controller 350 monitors the temperature of the cabinet cavity and runs the refrigeration system 320 to maintain the average actual temperature to equal the set point (e.g., the controller 350 runs the refrigeration system 320 when the cabinet cavity temperature is two degrees below the set point and shuts off the refrigeration system 320 when the cabinet cavity temperature is two degrees above the set point). The controller 350 logs the compressor runtime between defrost cycles and stores the compressor runtime in memory.

The plate evaporator 322 is defrosted by shutting down the refrigeration system 320 for a period time. The cooled, but not below freezing temperature, air in the cabinet cavity warms the plate evaporator 322 to melt any frost buildup on the plate evaporator 322. Additionally, the plate evaporator 322 is defrosted by the refrigerant that is equalizing in the refrigeration system 320. The plate evaporator 322 is not defrosted by hot refrigerant or an electric heater. The controller 350 determines when to defrost the plate evaporator 322 depending on the compressor runtime. The controller 350 calls for defrost after six (plate evaporator) or twelve hours (fin and tube evaporator) of compressor runtime. Typically, the cabinet cavity temperature will be greater than two degrees above the set point after defrost so that when the defrost ends, the controller 350 will run the refrigeration system 320 to cool the cabinet cavity. The longer the defrost time, the larger the increase in the cabinet cavity temperature. Therefore, the defrost time should be minimized to minimize the increase in the cabinet cavity temperature during defrost.

To minimize the defrost time, the controller 350 monitors the evaporator temperature T_(E) provided by the plate evaporator temperature sensor 354 and the elapsed defrost time. The controller 350 ends defrost when the evaporator temperature T_(E) reaches an evaporator temperature set point (e.g., 42° F.) or when a maximum defrost time (e.g., 90 minutes) has elapsed, whichever occurs first. The monitoring of the evaporator temperature T_(E) minimizes the defrost time and ensures that the plate evaporator 322 is properly defrosted. Limiting the defrost time to a maximum amount of time provides adequate defrosting while limiting the cabinet cavity temperature increase during defrost and prevents too much defrost time when the plate evaporator temperature sensor is not functioning properly. Additionally, when there has been a heavy buildup of frost (e.g., the door has been left ajar), limiting the defrost time prevents too much frost from melting at once so that the evaporator pan 93 does not overflow.

Referring now to FIG. 8, a user interface control unit 360 is mounted to the top of the refrigerator molded insert liner 306 within the cabinet 302 for receiving user commands and forwarding input signals to the main controller 350. The control unit 360 includes a display panel 362 and an input control board (not shown). The input control board is connected to the controller 350 and relays command information from a user inputted on a power input 366, a warmer input 368, a cooler input 370 and a light input 372. The controller 350 and control unit 360 are configured so that a user can set the minimum defrost time, the evaporator pan set point, and the maximum defrost time.

FIG. 11 shows a decision process 400 for deciding when to end a hot-gas defrost cycle or a plate defrost cycle. Beginning at the start of defrost 402, the length of the defrost time D_(T) is monitored and at decision block 404 it is determined whether the defrost time D_(T) is less than the maximum defrost time D_(T-MAX). If the defrost time D_(T) is not less than the maximum defrost time D_(T-MAX), then the defrost cycle is ended 406. If the defrost time D_(T) is less than the maximum defrost time D_(T-MAX), defrost continues and it is determined at decision block 408 whether the defrost temperature sensor temperature T_(D) (e.g., the temperature of the evaporator pan temperature sensor 142 or the plate evaporator temperature sensor 354) is below a defrost set point D_(SP) (e.g., 42° F.). If the defrost temperature sensor temperature T_(D) is not below the defrost set point D_(SP), then the defrost cycle is ended 406. If the defrost temperature sensor temperature T_(D) is below the defrost set point D_(SP), then decision blocks 404 and 408 are looped until the defrost time D_(T) is not less than the maximum defrost time D_(T-MAX) or the defrost temperature sensor temperature T_(D) is not below the defrost set point D_(SP). This method of controlling the defrost time can be adapted to be used with cooing units having any type of defrost method (e.g., hot gas defrost, plate defrost, strip heater defrost, or refrigerant flow reversal).

FIG. 12 shows the decision process 400 of FIG. 11 with an additional decision block 410. Decision block 410 is positioned after the NO output of decision block 404, after the NO output of decision block 408 and before end defrost cycle 406. At decision block 410, it is determined if the defrost time D_(T) is less than the minimum defrost time D_(T-MIN). If the defrost time D_(T) is less than the minimum defrost time D_(T-MIN), defrost continues and decision process 400 returns to decision block 404. If the defrost time D_(T) is not less than the minimum defrost time D_(T-MIN), the defrost is ended at 406. Decision block 410 is added to decision process 400 to ensure that the defrost cycle lasts at least for a minimum amount of time.

It should be appreciated that merely a preferred embodiment of the invention has been described above. However, many modifications and variations to the preferred embodiment will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiment. To ascertain the full scope of the invention, the following claims should be referenced. 

1. A cooling unit with a multi-parameter defrost control system, comprising: a refrigeration assembly including a compressor and an evaporator; a defrost temperature sensor that senses a defrost temperature; and a controller configured to control a defrost cycle of the refrigeration system, track an elapsed defrost time of the defrost cycle and monitor the defrost temperature, the controller programmed with a defrost set point and a maximum defrost time; wherein the controller is configured to end the defrost cycle when the elapsed defrost time has reached the maximum defrost time; wherein the controller is configured to end the defrost cycle when the defrost temperature has reached the defrost set point.
 2. The cooling unit of claim 1, further comprising a user input connected to the controller, wherein the defrost set point and the maximum defrost time can be set by the user input.
 3. The cooling unit of claim 1, wherein the refrigeration assembly includes a hot gas bypass valve configured to allow heated refrigerant to heat the evaporator when the controller opens the hot gas bypass valve during the defrost cycle.
 4. The cooling unit of claim 3, wherein the refrigeration assembly includes an evaporator pan and an evaporator pan heater that is energized by the controller during the defrost cycle to heat the evaporator pan.
 5. The cooling unit of claim 4, wherein the defrost temperature sensor is in thermal communication with the evaporator pan so that the defrost temperature indicates a temperature of the evaporator pan.
 6. The cooling unit of claim 5, wherein the defrost set point is 42 degrees Fahrenheit.
 7. The cooling unit of claim 5, wherein the controller is configured to continue the defrost cycle when the elapsed defrost time is less than a minimum defrost time.
 8. The cooling unit of claim 4, wherein the defrost temperature sensor is in thermal communication with the evaporator so that the defrost temperature indicates a temperature of the evaporator.
 9. The cooling unit of claim 8, wherein the controller is configured to not end the defrost cycle when the elapsed defrost time is less than a minimum defrost time.
 10. The cooling unit of claim 1, wherein the evaporator is a plate evaporator.
 11. The cooling unit of claim 10, wherein the defrost temperature sensor is in thermal communication with the plate evaporator so that the defrost temperature indicates a temperature of the plate evaporator.
 12. The cooling unit of claim 11, wherein the defrost set point is 42 degrees Fahrenheit.
 13. A method for controlling a defrost cycle of a cooling unit with a refrigeration assembly including a compressor and an evaporator, a defrost temperature sensor that senses a defrost temperature, and a controller programmed with a defrost set point and a maximum defrost time, the method comprising: monitoring the defrost temperature; tracking an elapsed defrost time of the defrost cycle beginning at a start time of the defrost cycle; ending the defrost cycle when the elapsed defrost time has reached the maximum defrost time; and ending the defrost cycle when the defrost temperature has reached the defrost set point.
 14. The method of claim 13, wherein the refrigeration assembly includes a hot gas bypass valve configured to allow heated refrigerant to heat the evaporator when the controller opens the hot gas bypass valve during the defrost cycle; wherein the refrigeration assembly further includes an evaporator pan and an evaporator pan heater that is energized during the defrost cycle to heat the evaporator pan; and wherein the defrost temperature sensor is in thermal communication with the evaporator pan so that the defrost temperature indicates a temperature of the evaporator pan.
 16. The method of claim 14; wherein the defrost temperature is monitored by a defrost temperature sensor that is in thermal communication with one of the evaporator and the evaporator pan.
 17. The method of claim 16, wherein the defrost set point is 42 degrees Fahrenheit.
 18. The method of claim 13, wherein the evaporator is a plate evaporator that is defrosted when the refrigeration system is not energized.
 19. The method of claim 18, wherein the defrost temperature is monitored by a defrost temperature sensor that is in thermal communication with the plate evaporator.
 20. The method of claim 19, wherein the defrost set point is 42 degrees Fahrenheit. 